leech

n.

"bloodsucking aquatic worm," from Old English læce (Kentish lyce), of unknown origin (with a cognate in Middle Dutch lake). Commonly regarded as a transferred use of leech (n.2), but the Old English forms suggest a distinct word, which has been assimilated to leech (n.2) by folk etymology [see OED]. Figuratively applied to human parasites since 1784.

For sense development, cf. Old Church Slavonic baliji "doctor," originally "conjurer," related to Serbo-Croatian bajati "enchant, conjure;" Old Church Slavonic vrači, Russian vrač "doctor," related to Serbo-Croatian vrač "sorcerer, fortune-teller." The form merged with leech (n.1) in Middle English, apparently by folk etymology. In 17c., leech usually was applied only to veterinary practitioners. The fourth finger of the hand, in Old English, was læcfinger, translating Latin digitus medicus, Greek daktylus iatrikos, supposedly because a vein from that finger stretches straight to the heart.

leech1 (lēch)n. Any of various chiefly aquatic bloodsucking or carnivorous annelid worms of the class Hirudinea, one species of which (Hirudo medicinalis) was formerly used by physicians to bleed patients. v.leeched, leech·ing, leech·es To bleed with leeches.